Pulsed radar systems have conventionally operated with a relatively low transmitter duty cycle determined by the maximum range to the target. As a result, the peak power required for the transmitter in order to obtain sufficient signal-to-noise ratio is high.
The duty cycle can be increased by increasing the pulse repetition frequency. This results in having several pulses in flight at the same time. In these circumstances, an apparatus must be provided for overcoming the ambiguous range problem due to multiple pulses in flight. For non-coherent radars, this commonly takes the form of random pulse position frequency jitter, which is effective for relatively small duty cycle improvement but degrades rapidly with increase in duty cycle.
Increasing pulse repetition frequency and overcoming ambiguity may be accomplished by encoding an RF pulse train which in turn will permit increasing the altimeter pulse repetition frequency by at least a factor of ten. This in turn will yield a corresponding increase in system loop sensitivity. Previous pulsed altimeter concepts require that the system wait for the ground echo from a given transmitted pulse to be received back at the receiver before transmitting another pulse. Doing so prevented the possibility of false "lock-on". Suitable encoding of a transmitted pulse sequence, and the corresponding receiver correlation of that code sequence, will permit many pulses to be in route to the ground and back at the same time. This increases the potential total average RF energy on the target, and improves sensitivity (particularly at high altitudes, where the "wait time" would previously have been relatively long). The transmitter pulse separation in these circumstances is now limited only by "pulse spreading" on the ground within the antenna beamwidth, and the codes ambiguous range rejection capabilities.
Heretofor, unresolved coding problem was not in generating the code but in the establishing of a simple method of variably delaying a complete code word in time (as opposed to a single pulse), relative to its origin, with virtually infinite resolution and over a wide range of delay times.